It is well known in the art to weld butt-jointed workpiece components using one or more laser beams to form a weld seam between the butt-jointed workpiece components. Many factors influence the quality of the resulting weld seam. The influence of various welding parameters on the seam flank angle α of a laser beam weld seam only becomes apparent on the completed welded workpiece, for example when inspecting the laser weld seam for quality control purposes. At that time, it is often discovered that the laser weld seam does not meet the quality requirements or expectations. When it is discovered in the subsequent quality control measures, that a laser welded workpiece does not meet the quality requirements, it is often necessary to re-work or especially re-weld at least portions of the laser welded seam of a rejected workpiece. That significantly increases the technological costs of a finished welded component on a per-piece basis. Particularly, such re-welding work is carried out on a previously laser welded component when it is subsequently determined that the weld seam angle α lies outside of the acceptable tolerance range. Therefore, in order to minimize or limit the reject rate of laser welded workpieces, which have been welded with conventionally known laser welding equipment, there is a need for an intensive monitoring of the seam flank angle α during the laser beam welding of workpieces such as stringer-skin components in the manufacture of aircraft fuselages.
The weld seam angle α is regarded as an essential basic starting value or initial value for the laser welding of welded components such as a stringer-skin arrangement in the manufacture of aircraft fuselages, because it is known that the resulting weld seam angle α has an influence on the quality of the laser weld seam in the finished welded part. Traditionally, however, the evaluation of the weld seam angle α is only carried out after completion of the welding of the entire weld seam. On the other hand, solutions that can have a direct influence on the welding process itself, i.e. during the welding process, to achieve an adjustment or variation of the weld seam angle α being formed, in order to achieve a qualitatively acceptable weld seam, are not known. In this context, the primary welded components of importance involve stringers standing perpendicularly on a skin sheet, also known as a “flat sheetmetal shell” in the art of aircraft fuselage manufacture. The final weld seam angle α results from the position of the laser beam and its relative location and orientation with respect to the welded butt of the parts to be joined, for example the skin-stringer butt. In this regard, the weld seam angle α is the angle lying between the plane of the sheetmetal skin and the finished surface of the weld seam or weld bead. The resulting weld seam angle α is influenced by several factors, whereby an influence on the seam position and in correlation to the expected seam flange angle α of the finished weld seam can already arise before the beginning of the welding process.
Previously, the position and orientation of the laser beam relative to the weld butt joint of the parts (e.g. the skin-stringer butt) has been fixedly prescribed before the laser welding process is carried out. With such a preparation for the laser welding process, the prescribed laser beam position, e.g. location and orientation, is based on a theoretically expected seam position with respect to a seam flank angle of the welded seam.
One must further consider that several factors arising only during the welding process can also have an unintended and undesired influence on the further performance of the welding process. Such unforeseeable interferences, for example involve or arise from the shifting, straining, or tilting of the workpiece components to be welded, for example the stringer on the skin, the variation of the position tolerances of the laser beam, tolerances of the workpiece thickness that have not been taken into consideration, e.g. the stringer thickness, tolerances and variations arising from torsion and bending of the workpiece through a roll guidance thereof, and tolerances of a tactile optical sensor for sensing or monitoring the butt joint. When such factors have an unintended influence on the welding process during the performance of the welding process itself, there is no known further possibility by which one could have a decisive or effective influence on the formation of the seam position or on the fluctuations of the seam flank angle α during the occurring laser welding process, in order to achieve a corresponding high workpiece quality of the laser welded parts.
It is especially difficult and problematic to maintain the prescribed tolerance limits for the seam flank angle α when laser welding spherically curved skin sheets or fields. Also, the great amount of time required for the proper adjustment and set up for welding each stringer on such a spherically curved skin field will add further difficulty and expense to the often required re-welding to correct finished weld seams of sub-standard quality.
Conventionally, it is known to carry out an optical monitoring and measuring of the resulting weld seam after completion of the laser welding of the parts to be joined, for example the skin-stringer joint. Such optical inspection and measuring is conventionally carried out, for example, with known light section processes using linear light beam or light line projection, for example by means of a known light section sensor, i.e. a so-called Falldorf sensor produced and sold by the Falldorf & Co. GmbH company (Falldorf & Co. GmbH, Consulting & Engineering, Fahrenheit Strasse 1, 28359 Bremen, Germany or at the internet address http://www.falldorf.de). In this manner, the laser welded joint can be evaluated with respect to the requirements relating to the weld seam quality. Such a welding apparatus, with such a subsequent optical weld seam quality inspection arrangement, is based on the theoretically expected seam position or particularly the expected seam flank angle α of a laser weld seam which is determined with consideration of known values based on prior experience. With such conventionally known measures, the welding process cannot be positively influenced during the performance of the welding process itself, because the known measures do not provide for the regulation of the seam position or especially the seam flank angle α during the welding process, but rather only inspection and evaluation of these weld seam features after completion of the welding process.
It has also previously been known to use a mechanical sensor that mechanically senses or detects the position of the butt joint such as the stringer-skin butt at a position running ahead or spaced ahead of the welding point, in order to correct the initially prescribed seam position. It has been found in practice, that this arrangement using a mechanical sensor running ahead of the weld point does not always lead to acceptable seam flank angles α in the finished welded product. For example, the above mentioned interference values that arise during the welding process cannot be adequately compensated for using such an arrangement with a mechanical sensor.
Furthermore, the German Patent Laying-Open Document DE 42 16 643 A1, and especially FIGS. 2, 8, and 22 thereof, disclose an apparatus for regulating the seam position of a laser welded profile or sectional component, especially involving the edge surface of a first component butting perpendicularly against the upper surface of a second sectional component, whereby these two components are welded together to form a single welded workpiece by means of two weld seams along the edge of the butt joint on opposite sides of the first butting component, whereby each of these weld seams is respectively formed by a welding laser beam.
According to the known apparatus and method disclosed by DE 42 16 643 A1, and especially FIG. 7 in connection with col. 9, lines 30 et. seq., the angle at which the laser beams meet or impinge on the workpiece outer surface, and which influences the seam position, is regulated dependent on the measuring results of a measuring instrument. Thus, the apparatus disclosed therein apparently comprises several system components that are serially arranged and connected to each other in an information technical manner, i.e. for data exchange. The system components especially include a seam position component for varying the seam position, a laser welding processing component with two integrated welding laser beam sources as well as a seam position regulating arrangement. Moreover, a sensor arrangement for sensing or detecting characteristic values of the laser welded seam is arranged near the laser welding process component. The sensor arrangement, and particularly the respective sensor, is arranged to run along after or with the welding laser beam sources that move along with the welding speed along the welding seam (see claim 12 of DE 42 16 643 A1). The sensor arrangement is connected for information exchange with the seam position regulating arrangement (see DE 42 16 643 A1 at FIG. 8 in connection with col. 9, lines 42 et. seq., as well as FIG. 22 in connection with col. 12, line 10 et. seq), whereupon the seam position regulating arrangement receives or picks up the sensor information that has been converted to corresponding sensor signals. In this regard, the seam position regulating arrangement is apparently internally equipped with a circuit that carries out an information comparison of the sensor informations that are derived externally from the circuit, but delivered to the circuit during the welding process, with a nominal or desired value information. Then, after the information comparison, based on the result of the comparison, the circuit generates a regulating value for the process parameters, which are provided at the output of the seam position regulating arrangement and delivered to the seam position component for varying the seam position.
The above described known prior art does not disclose and would not have suggested an arrangement and a method for regulating the seam position of a laser beam welded sectional workpiece, using a sensor arrangement for detecting or sensing the weld seam angle during the performance of the welding operation, in order to adjust the welding parameters “on the fly” still during the welding operation, to thereby correctively or adjustingly influence the weld position of the weld seam as it is being formed.